Evolution of Voids and Fractures in Wellbore Cement under Dynamic Flow Conditions Relevant to Geological Carbon Sequestration

Cao, Peilin; Karpyn, Zuleima; Li, Li

Evolution of Voids and Fractures in Wellbore Cement under Dynamic Flow Conditions Relevant to Geological Carbon Sequestration Interaction between wellbore cement and injected CO2 is one of the main processes that can potentially alter cement properties. Cracks/fractures inside cement plugs or weak bonding between formation/cement, may form preferential leakage pathways due to cement alteration during geological carbon sequestration. This work investigates the evolution of cement properties under dynamic flow conditions. A cement casted sandstone core of 3.81cm diameter and 7.67cm long, with voids and fractures in the central cemented region was made. The cemented sandstone core was flooded continuously using CO2-saturated brine under pressure and temperature conditions relevant to carbon sequestration. Changes in cement and void space were visualized and quantified using X-ray Micro-CT imaging. Within an eight-day flow-through period, the cement zone experienced chemical alterations evident through color changes as well as changes in fracture volume, surface area, and structure while the host sandstone remained unaltered. A higher rate of cement degradation was observed in the early stage of CO2-saturated brine flooding. The rate of cement alteration varies spatially due to brine acidity variation along the core, such that fracture apertures located near the core inlet experienced more severe cement degradation than those closer to the core outlet, by a factor of two. Analysis of fracture growth rate shows larger initial fracture apertures correlated with smaller relative aperture growth due to the relatively smaller flow velocity. Fracture rims were observed to become smoother from CT images, which were confirmed by decreasing surface area to volume ratio over time. This may be due to carbonate calcium precipitation as a result of carbonation during the flow-through experiment. In addition, in-situ permeability of a parallel fractured cement-casted sample was measured to increase by a factor of 24 over 10 days of continuous injection of CO2-saturated brine. This work provided mechanistic and quantitative understanding of the detailed temporal and spatial evolution of cement alteration under carbon sequestration conditions. Findings from this work will help to identify the conditions that may threaten seal integrity and provide valuable insights for the development of predictive models for risk assessment under conditions relevant to carbon sequestration.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013